1. Field of Invention
The invention relates to an amplifier, and particularly to a variable gain amplifier.
2. Related Art
One of the major challenges of the Wideband Code Division Multiple Access (WCDMA) systems is a need for an accurate linear-to-dB gain control over 74-dB of gain tuning range. For a WCDMA transmitter (TX), the two system architectures as shown in FIG. 1 are commonly used: direct-conversion 100, and 2-stage conversion 150. Compared to the 2-stage conversion 150, the direct conversion 100 provides a better solution from both image (sideband) rejection and power saving perspectives. The output of a mixer includes a desired signal and an undesired sideband. However, the gain control requirements for the high accuracy and the wide tuning range hinder the popularity of the direct conversion. Consider the direct conversion architecture 100 FIG. 1a that is used to provide a gain control range of 90-dB. The IQ baseband inputs are received by baseband variable gain low pass filters 102, modulated by radio frequency (RF) signals at RF modulation mixes 104, 106, and finally pass through a RF variable gain amplifier (RF VGA) 108. The gain control is usually shared between the IQ baseband low pass filters (LPF) 102 and the RF VGA 108. For RF VGA 108, the maximum gain control range is approximately 30-dB due to limited device isolation at RF VGA 108. Therefore, at least 60-dB gain control has to be assigned to the LPFs 102. This sets a very strict local oscillator (LO) leakage requirement to the IQ mixers 106, which must have at least 80-dB of LO rejection in order to achieve −20 dBc carrier level at minimum gain setting. Carrier leakage calibration techniques can be use, but most of the techniques require a very accurate and sensitive RF detector and complex digital signal processor (DSP), making the direct conversion a less attractive solution.
To alleviate the high gain control in the IQ LPF, an architecture using a intermediate frequency (IF) VGA as shown in FIG. 1b can be added to provide extra gain control and more accurate gain tuning. Furthermore, this architecture also resolves the LO leakage problem by external filtering. In FIG. 1b, the IQ baseband inputs are received by baseband variable gain low pass filters 154, modulated by intermediate frequency (IF) signals at IF modulation mixes 156, 158, and finally pass through an IF variable gain amplifier (IF VGA) 160. After the IF VGA 160, a second stage conversion begins with a RF up-conversion mixer 162 where the signal from the first stage is mixed with the RF, and then the result passes through a RF VGA 162. Consider the nodes before and after the RF mixer 162 (node X and Y respectively), and the corresponding signals at these two nodes are shown in FIG. 2, where the carrier is represented by an arrow 202 and the signal is represented by a triangle 204. After the IF VGA at X, both the signal 204 and the carrier 202 scale according to the VGA gain, and the IF LO rejection remains the same as the attenuation occurs after the IF mixing stage. The LO rejection is the difference between the signal and the carrier leakage and the VGA after the RF mixer 162 attenuates both the signal and the carrier leakage by the same amount, so LO rejection remains the same. For example, for VGA gain=−10 dB, signal=10 dBm, carrier=−10 dBm:
without the VGALO rejection = 10 dBm − (−10 dBm, carrier) =20 dBIf the VGA is placedLO rejection = 10 dBm − 10 dB (due to VGA) −before the mixer(−10 dBm, carrier power) = −10 dBmif the VGA is placedLO rejection = 10 dBm − 10 dB (due to VGA) −after the mixer[(−10 dBm, carrier) − 10 dB (due to VGA)] =20 dB
The result is the same as in the first case because both signal and carrier are attenuated by the VGA.
A RF mixer 162 will introduce a RF LO tone, which is one IF away from the signal, since if the frequency at the mixer is IF and the output will be LO+IF and LO−IF. By choosing a wide enough IF (e.g. 400-MHz), the RF LO tone can be removed by an external SAW filter before the power amplifier. In theory, all the gain control can be assigned to the IF VGA, but this will require the RF driver to have a very low noise performance. As a consequence, variable gain assignment will still be assigned across all three stages (LPF, IF VGA and RF VGA) in practical implementation, which makes tuning difficult.
Another two commonly used variable gain topologies are: (a) translinear cell (FIG. 3) and (b) current steering circuit (FIG. 4). However, both topologies have relatively poor performance in terms of isolation at high frequency. Isolation is an important consideration because the total VGA tuning range is 90-dB, and therefore at least 90-dB of isolation is required across the TX chain. Typical device reverse isolation is approximately 30-dB for high frequency devices (RF devices), making circuits as shown in FIG. 3 and FIG. 4 less attractive for RF applications due to limited isolation.
Therefore, it is to a RF VGA that enables an accurate linear gain tuning range without increasing the isolation problem the present invention is primarily directed.